Rolled steel bar or wire rod for hot forging

ABSTRACT

A rolled steel bar or wire rod having hot surface fatigue strength, wear resistance, and machinability even after hot forging has a composition containing C, Si, Mn, S, Cr, Mo (optional), Al, and N, with the balance being Fe and impurities. The chemical composition satisfies that fn1 defined by Formula (1) is 1.60 to 2.10. The structure of the rolled steel bar or wire rod for hot forging includes a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. A maximum value/a minimum value of average ferrite grain size, which is observed and measured randomly in 15 visual fields each having an area of 62500 μm 2  in a cross section, is not more than 2.0. fn1=Cr+2×Mo (1), and each symbol of elements in Formula (1) is substituted by a content (mass %) of a corresponding element.

TECHNICAL FIELD

The present invention relates to a steel bar or wire rod, and more specifically relates to a rolled steel bar or wire rod for hot forging.

BACKGROUND ART

Machine parts such as gears and pulleys are used for automobiles or industrial machinery. Majority of these machine parts are produced by the following method. A starting material composed of alloy steel for machine structural use is prepared. The starting material has, for example, a chemical composition corresponding to SCr420, SCM420, or SNCM 420 in Japanese Industrial Standard (JIS). The starting material is, for example, a hot rolled steel bar or wire rod. The starting material is subjected to hot forging to produce an intermediate product. The intermediate product is subjected to normalizing as needed. Further, the intermediate product is subjected to cutting. The intermediate product after cutting is subjected to a casehardening treatment. The casehardening treatment may be, for example, carburizing quenching, carbonitriding quenching, or induction quenching. The casehardened intermediate product is subjected to tempering at a tempering temperature of not more than 200° C. The intermediate product after tempering is subjected to shotpeening as needed. Through the above described processes, machine parts are produced.

In recent years, to cope with increases in the fuel efficiency of automobiles and the output power of engines, machine parts have been reduced in weight and size. Thus, loads imposed on machine parts have increased compared with the past. For that reason, those machine parts are required to have excellent bending fatigue strength, surface fatigue strength (contact fatigue strength), and wear resistance.

On the other hand, the reduction of production cost of machine parts is also demanded. Specifically, omission of additional processes such as shotpeening is demanded to reduce production cost. Further, the proportion of the cost of cutting in the production cost is large. Therefore, to reduce production cost, high machinability is required for rolled steel bar or wire rod for hot forging, which is to be used for the starting material for machine parts.

Accordingly, for a rolled steel bar or wire rod for hot forging which provides the starting material for machine parts, excellent machinability is required in addition to excellent bending fatigue strength, surface fatigue strength and wear resistance.

Techniques to improve the properties of steel to be used as the starting material for machine parts are proposed in JP60-21359A, JP7-242994A, and JP7-126803A.

In steel for gears disclosed in JP60-21359A, Si and P contents are specified as Si: not more than 0.1% and P: not more than 0.01%. JP60-21359A describes that such specification allows the steel for gear to have high strength, as well as toughness and high reliability.

Steel for gears disclosed in JP7-242994A contains Cr: 1.50 to 5.0%, and further contains, as needed, Si: 0.40 to 1.0% while satisfying 7.5%>2.2×Si(%)+2.5×Mn (%)+Cr (%)+5.7×Mo (%). JP7-242994A describes that having such a chemical composition allows the steel for gear to have excellent tooth surface strength.

Steel for carburized gears disclosed in JP7-126803A contains Si: 0.35 to not more than 3.0%, V: 0.05 to 0.5%, etc. JP7-126803A describes that, having such a chemical composition, the steel for gear can have a high bending fatigue strength and a high surface fatigue strength.

DISCLOSURE OF THE INVENTION

However, in JP60-21359A, no investigation has been made on the surface fatigue strength. For that reason, the steel for gear disclosed in JP60-21359A may have low surface fatigue strength. In JP7-242994A, no investigation has been made on the bending fatigue strength. For that reason, the steel for gear disclosed in JP7-242994A may have low bending fatigue strength. The steel for gear disclosed in JP7-126803A contains V. V increases the hardness of steel after hot forging. For that reason, the steel after hot forging may have reduced machinability. In short, none of JP60-21359A, JP7-242994A, and JP7-126803A discloses a steel having excellent bending fatigue strength, surface fatigue strength, and wear resistance, as well as excellent machinability.

It is an object of the present invention to provide a rolled steel bar or wire rod for hot forging which has excellent bending fatigue strength, surface fatigue strength, wear resistance, and machinability even after hot forging.

A rolled steel bar or wire rod for hot forging according to the present invention has a chemical composition comprising, by mass %, C: 0.1 to 0.250, Si: 0.30 to 0.60%, Mn: 0.50 to 1.0%, S: 0.003% to 0.05%, Cr: 1.50 to 2.00%, Mo: not more than 0.10% (including 0%), Al: 0.025 to 0.05%, N: 0.010 to 0.025%, and the balance being Fe and impurities, wherein the impurities contain P: not more than 0.025%, Ti: not more than 0.003%, and O (oxygen): not more than 0.002%, respectively, and wherein fn1 defined by Formula (1) is 1.60 to 2.10. The above described rolled steel bar or wire rod for hot forging has a structure consisting of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. A maximum value/a minimum value of average ferrite grain size, which is obtained by making a measurement in 15 visual fields each having an area of 62500 μm² in a cross section, is not more than 2.0.

fn1=Cr+2×Mo  (1)

where each symbol of elements in Formula (1) is substituted by a content (mass %) of a corresponding element.

The steel bar or wire rod for hot forging according to the present invention has excellent bending fatigue strength, surface fatigue strength, wear resistance, and machinability.

The rolled steel bar or wire rod for hot forging according to the present invention may contain Nb: not more than 0.08% by mass % in place of a part of Fe.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a small roller specimen for a roller pitting test, which is fabricated in Examples.

FIG. 2 is a side view of a notched Ono-type rotating bending fatigue specimen, which is fabricated in Examples.

FIG. 3 is a diagram showing a carburizing quenching condition in Examples.

FIG. 4 is a front view of a large roller for a roller pitting test in an Example.

DESCRIPTION OF EMBODIMENTS

The present inventors have conducted investigation and research on bending fatigue strength, surface fatigue strength, wear resistance, and machinability of a rolled steel bar or wire rod for hot forging (hereafter, simply referred to as a steel bar or wire rod). As a result, the present inventors have obtained the following findings.

(a) As the Si content increases, the surface fatigue strength and the wear resistance of steel increases. Further, as the Cr content and the Mo content increase, the bending fatigue strength, the surface fatigue strength, and the wear resistance of steel increase.

(b) On the other hand, if the Mo content is excessively high, production of bainite is promoted in steel after hot forging, and in steel after subjected to hot forging and further to normalizing. Similarly, even when Mo is not contained, if the Cr content is excessively high, production of bainite is promoted. Bainite deteriorates the machinability of steel. Therefore, it is preferable to suppress the production of bainite, thereby suppressing the deterioration of the machinability of steel.

(c) As so far described, to obtain excellent bending fatigue strength, surface fatigue strength and wear resistance as well as excellent machinability, it is preferable to adjust Si, Mo, and Cr contents. Particularly, to increase machinability while increasing bending fatigue strength, surface fatigue strength and wear resistance, it is preferable to adjust the total amount of Cr and Mo contents. Specifically, if the chemical composition of steel satisfies Formula (2), it is possible to obtain excellent bending fatigue strength, surface fatigue strength, wear resistance, and machinability:

1.60≦Cr+2×Mo≦2.10  (2)

where, each symbol of elements in Formula (2) is substituted by the content (mass %) of a corresponding element.

(d) If the variation of the grain size in a steel bar or wire rod is large, the bending fatigue strength will decline. When the variation of grain size is large, the surface fatigue strength may also decline. As an index to show the degree of variation of grain size, a ratio of average ferrite grain size is defined as follows. Fifteen visual fields each having an area of 62500 μm² are selected from a region excluding any decarburized layer in the outer layer of a cross section of the steel bar or wire rod. Image analysis is performed on each of the selected 15 visual fields. Specifically, an average ferrite grain size is measured in each visual field. The average ferrite grain size in each visual field is measured according to the intercept method specified in JIS G0551 (2005).

Out of the average ferrite grain sizes determined in each of the 15 visual fields, a maximum value and a minimum value are selected. Then the maximum value/the minimum value is determined. The determined value is defined as a ratio of average ferrite grain size. That is, the ratio of average ferrite grain size is defined by the following Formula (3):

A ratio of average ferrite grain size=the maximum value among average ferrite grain sizes obtained in 15 visual fields/the minimum value among average ferrite grains sizes obtained in 15 visual fields. (3)

When the ratio of average ferrite grain size is not more than 2.0, the variation of grains in steel is small. As a result, the bending fatigue strength and surface fatigue strength of steel are high.

The rolled steel bar or wire rod for hot forging according to the present invention has been completed based on the above described findings. Hereafter, the rolled steel bar or wire rod for hot forging according to the present invention will be described in detail. Hereafter, “%” indicating the content of an element constituting a chemical composition represents “mass %”.

[Chemical Composition]

The chemical composition of the steel bar or wire rod according to the present invention contains the following elements.

C: 0.1 to 0.25%

Carbon (C) increases carburizing-quenching or carbonitriding-quenching hardenability. Therefore, C increases the strength of steel. Particularly, C increases the strength of the core portion of a machine part after carburizing and quenching or carbonitriding and quenching. On the other hand, when C is excessively contained, the amount of deformation of a machine part after carburizing and quenching or carbonitriding and quenching will remarkably increase. Therefore, the C content shall be 0.1 to 0.25%. The lower limit of the C content is preferably more than 0.1%, more preferably not less than 0.15%, and further preferably not less than 0.18%. The upper limit of the C content is preferably less than 0.25%, more preferably not more than 0.23%, and further preferably not more than 0.20%.

Si: 0.30 to 0.60%

Silicon (Si) improves the hardenability of steel. Si further improves the temper softening resistance of steel. Therefore, Si increases the surface fatigue strength and wear resistance of steel. On the other hand, if Si is excessively contained, the strength of steel after hot forging will become excessively high. As a result, the machinability of steel will deteriorate. Further, if Si is excessively contained, the bending fatigue strength will decline. Therefore, the Si content shall be 0.30 to 0.60%. The lower limit of the Si content is preferably more than 0.30%, more preferably not less than 0.40%, and further preferably not less than 0.45%. The upper limit of the Si content is preferably less than 0.60%, more preferably not more than 0.57%, and further preferably not more than 0.55%.

Mn: 0.50 to 1.0%

Manganese (Mn) improves the hardenability of steel and increases the strength of steel. Therefore, Mn increases the strength of the core portion of a machine part subjected to carburizing and quenching, or carbonitriding and quenching. On the other hand, if Mn is excessively contained, the machinability of steel after hot forging will deteriorate. Further, when Mn is excessively contained, Mn oxide will be produced on the surface of steel. That will result in an increase in the depth of an abnormally carburized layer after carburizing and quenching, or carbonitriding and quenching. The abnormally carburized layer is, for example, a boundary oxidation layer and a slack quenched layer. As the depth of the abnormally carburized layer increases, the bending fatigue strength and the pitting strength of steel will decline. Pitting is one form of fracture due to surface fatigue. Therefore, as the pitting strength declines, the surface fatigue strength declines as well. Therefore, the Mn content shall be 0.50 to 1.0%. The lower limit of the Mn content is preferably more than 0.50%, more preferably not less than 0.55%, and further preferably not less than 0.60%. The upper limit of the Mn content is preferably less than 1.0%, more preferably not more than 0.95%, and further preferably not more than 0.9%.

S: 0.003 to 0.05%

Sulfur (S) combines with Mn to form MnS. MnS improves the machinability of steel. On the other hand, if S is excessively contained, coarse MnS will be formed. Coarse MnS decreases the bending fatigue strength and the surface fatigue strength of steel. Therefore, the S content shall be 0.003 to 0.05%. The lower limit of the S content is preferably more than 0.003%, further preferably not less than 0.005%, and further preferably not less than 0.01%. The upper limit of the S content is preferably less than 0.05%, more preferably not more than 0.03%, and further preferably not more than 0.02%.

Cr: 1.50 to 2.00%

Chromium (Cr) improves the hardenability of steel and the temper softening resistance of steel. As a result, Cr increases the bending fatigue strength, the surface fatigue strength, and the wear resistance of steel. On the other hand, if Cr is excessively contained, the production of bainite is promoted in steel after hot forging or after normalizing. As a result, the machinability of steel deteriorates. Therefore, the Cr content shall be 1.50 to 2.00%. The lower limit of the Cr content is preferably more than 1.50%, more preferably not less than 1.70%, and further preferably not less than 1.80%. The upper limit of the Cr content is preferably less than 2.00%, more preferably not more than 1.95%, and further preferably not more than 1.90%.

Mo: Not More than 0.10% (Including 0%)

Molybdenum (Mo) may not be contained. Mo improves the hardenability and the temper softening resistance of steel. As a result, Mo increases the bending fatigue strength, the surface fatigue strength, and the wear resistance of steel. On the other hand, if Mo is excessively contained, the production of bainite is promoted in steel after hot forging or normalizing. As a result, the machinability of steel deteriorates. Therefore, the Mo content shall be not more than 0.10% (including 0%). The lower limit of the Mo content is preferably not less than 0.02%. The upper limit of the Mo content is preferably less than 0.10%, and more preferably not more than 0.08%, and further preferably not more than 0.05%.

Al: 0.025 to 0.05%

Aluminum (Al) deoxidizes steel. Further, Al combines with N to form AlN. AlN suppresses the coarsening of austenitic grains due to carburizing and heating. On the other hand, if Al is excessively contained, Al forms coarse Al oxides. Coarse Al oxides decrease the bending fatigue strength of steel. Therefore, the Al content shall be 0.025 to 0.05%. The lower limit of the Al content is preferably more than 0.025%, more preferably not less than 0.027%, and further preferably not less than 0.030%. The upper limit of Al is preferably less than 0.05%, more preferably not more than 0.045%, and further preferably not more than 0.04%.

N: 0.010 to 0.025%

Nitrogen (N) combines with Al or Nb to form AlN or NbN. AlN or NbN suppresses the coarsening of austenitic grains due to carburizing and heating. On the other hand, if N is excessively contained, stable production in steel making process becomes difficult. Therefore, the N content shall be 0.010 to 0.025%. The lower limit of the N content is preferably more than 0.010%, more preferably not less than 0.012%, and further preferably not less than 0.013%. The upper limit of N is preferably less than 0.025%, more preferably not more than 0.020%, and further preferably not more than 0.018%.

The balance of the chemical composition of the steel bar or wire rod according to the present invention is comprised of Fe and impurities. The impurities as used herein represent elements which are mixed from ores and scrap to be used as the raw material of steel, or the environments of production processes. In the present invention, the contents of P, Ti, and 0 (oxygen) as impurities are limited as follows.

P: Not More than 0.025%

Phosphorus (P) segregates at grain boundaries and embrittles the grain boundaries. As a result, P decreases the fatigue strength of steel. Therefore, the P content is preferably as low as possible. The P content shall be not more than 0.025%. The P content is preferably less than 0.025%, and more preferably not more than 0.020%.

Ti: Not More than 0.003%

Titanium (Ti) combines with N to form coarse TiN. Coarse TiN decreases the fatigue strength of steel. Therefore, the Ti content is preferably as low as possible. The Ti content shall be not more than 0.003%. The Ti content is preferably less than 0.003%, and further preferably not more than 0.002%.

O (Oxygen): Not More than 0.002%

Oxygen (O) combines with Al to form oxide based inclusions. Oxide based inclusions decrease the bending fatigue strength of steel. Therefore, the 0 content is preferably as low as possible. The 0 content shall be not more than 0.002%. The 0 content is preferably less than 0.002%, and more preferably not more than 0.001%.

The chemical composition of the steel bar or wire rod according to the present invention satisfies Formula (2).

1.60≦Cr+2×Mo≦2.10  (2)

Where, each symbol of elements in Formula (2) is substituted by the content (mass %) of a corresponding element.

As so far described, both Cr and Mo improve the hardenability and the temper softening resistance of steel. As a result, Cr and Mo increase the bending fatigue strength, the surface fatigue strength, and the wear resistance of steel. Comparing Mo with Cr, Mo achieves the same level of advantageous effects (increases in the bending fatigue strength, the surface fatigue strength, and the wear resistance) as Cr by an amount half of the Cr content. Therefore, it is defined that fn1=Cr+2Mo. Each symbol of elements in fn1 is substituted by the content (mass %) of a corresponding element (Cr or Mo).

When fn1 is less than 1.60, at least one or more kinds of the bending fatigue strength, the surface fatigue strength and the wear resistance of steel will decline. On the other hand, when fn1 exceeds 2.10, the production of bainite is promoted in steel after hot forging or normalizing. As a result, the machinability of steel deteriorates. When fn1 is 1.60 to 2.10, it is possible to increase the bending fatigue strength, the surface fatigue strength, and the wear resistance of steel while suppressing the deterioration of the machinability of steel. The lower limit of fn1 is preferably not less than 1.80. The upper limit of fn1 is preferably less than 2.00.

The chemical composition of the rolled steel bar or wire rod for hot forging according to the present invention may contain Nb in place of a part of Fe.

Nb: Not More than 0.08%

Niobium (Nb) is a selective element. Nb combines with C and N to form Nb carbides, Nb nitrides or Nb carbonitrides. Nb carbides, Nb nitrides and Nb carbonitrides suppress, as with Al nitrides, the coarsening of austenitic grains during carburizing and heating. If even a slight amount of Nb is contained, the above described effect will be achieved. On the other hand, when Nb is excessively contained, Nb carbides, Nb nitrides and Nb carbonitrides will become coarse. For that reason, it is not possible to suppress the coarsening of austenitic grains during carburizing and heating. Therefore, the Nb content shall be not more than 0.08%. The lower limit of the Nb content is preferably not less than 0.01%. The upper limit of the Nb content is preferably less than 0.08%, and more preferably not more than 0.05%.

[Microstructure]

The microstructure of the steel bar or wire rod according to the present invention consists of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. Here, the “ferrite-pearlite structure” represents a two-phase structure whose matrix (parent phase) consists of ferrite and pearlite. The “ferrite-pearlite-bainite structure” represents a three-phase structure whose matrix consists of ferrite, pearlite, and bainite. The “ferrite-bainite structure” represents a two-phase structure whose matrix consists of ferrite and bainite.

In short, the microstructure of the steel bar or wire rod according to the present invention does not contain martensite. Martensite is hard, and it deteriorates the ductility of steel. Therefore, when a steel bar or wire rod containing martensite is conveyed or straightened, the steel bar or wire rod is likely to have cracks. Since the microstructure of the steel bar or wire rod according to the present invention does not contain martensite, cracks are not likely to occur during straightening or conveyance.

Each phase described above is identified by the following method. A sample is cut out which includes a central portion of a section (cross section) perpendicular to the longitudinal direction of the steel bar or wire rod. The surface (including the central portion) of the cut out sample is mirror polished. The polished surface is etched with Nital. The etched surface is subjected to microstructure observation by an optical microscope of 400-times magnification. Specifically, 15 visual fields are arbitrarily selected from a region in the etched surface excluding any decarburized layer of the outer layer of the steel bar or wire rod. Then, each visual field is observed to identify the microstructure. If bainite is included in any of the 15 visual fields, it is judged that the microstructure of the steel includes bainite. Similar judgment is made for ferrite and pearlite as well. The size of each visual field is 250 μm×250 μm=62500 μm².

Further, in the above described microstructure, the ratio of average ferrite grain size defined by Formula (3) is not more than 2.0, in a cross section.

Image analysis is performed on each of the above described 15 visual fields. Specifically, a ferrite phase is identified in each visual field. The ferrite grain size in the identified ferrite phase is measured. An average ferrite grain size of each visual field is measured according to the intercept method specified in JIS G0551 (2005).

Among average ferrite grain sizes (15 in total) determined in each of the 15 visual fields, the maximum and the minimum values are selected. Then, based on the above described Formula (3), the ratio of average ferrite grain size=(the maximum value of ferrite average grains size/the minimum value of average ferrite grain size) is determined.

When the grain size is non-uniform in a steel material after hot rolling (that is, an as-hot rolled material), the grain size will remain to be non-uniform even after hot forging or carburizing and quenching which is a post process. If the grain size is non-uniform, the bending fatigue strength and the surface fatigue strength will decline. Therefore, the grain size in the as-hot rolled material is preferably as uniform as possible. To evaluate the degree of uniformity of grain size, it is preferable to evaluate the ratio of average ferrite grain size. The ferrite grain size can be observed easier than that of pearlite or bainite by means of etching. Therefore, investigating the degree of uniformity of average ferrite grain size (that is, the ratio of average ferrite grain size) facilitates the evaluation of the degree of uniformity of the grain size in structure. Further, fatigue fracture occurs starting from a location of lowest strength. For that reason, using the maximum value/the minimum value of average ferrite grain size rather than the standard deviation of average ferrite grain size as an index is more suitable for evaluating the bending fatigue strength and the surface fatigue strength.

If the microstructure consists of various mixed structures including ferrite as described above and the ratio of average ferrite grain size is not more than 2.0, the variation of grain size in the steel bar or wire rod is small. As a result, the bending fatigue strength and the surface fatigue strength of steel after hot forging, or carburizing and quenching increase. The ratio of average ferrite grain size is preferably not more than 1.6.

On the other hand, if the ratio of average ferrite grain size exceeds 2.0, one or more kinds of the bending fatigue strength and the surface fatigue strength of steel will decline.

[Production Method]

An example of the method for producing the steel bar or wire rod according to the present invention, and an example of the method for producing a machine part represented by a gear and a pulley will be described. Note that the production method will not be limited to the following.

Molten steel having the above described chemical composition and satisfying Formula (2) is produced. The molten steel is used to produce a cast piece (slab or bloom) by a continuous casting process. In the continuous casting process, the cast piece in the course of solidification is subjected to rolling reduction. Next, the cast piece is heated. The heating temperature in this occasion is 1250 to 1300° C. and the heating time is not less than 10 hours. The heated cast piece is billeted by a billeting machine to produce a billet.

The billet is hot rolled to produce a steel bar or wire rod for hot forging. Specifically, the billet is heated. In this occasion, the heating temperature is 1150 to 1200° C., and the heating time is not less than 1.5 hours. The heated billet is hot rolled to produce a steel bar or wire rod. The finishing temperature in the hot rolling is 900 to 1000° C. Water cooling is not performed before finish rolling. After the finish rolling, the steel bar or wire rod is cooled at a cooling rate not more than that of spontaneous cooling in the air (hereafter, simply referred to as spontaneous cooling) until the surface temperature reaches not more than 600° C. In the hot rolling, a reduction of area (%), which is defined by Formula (4), shall be not less than 87.5%:

Reduction of area={1−(sectional area of steel bar or wire rod/sectional area of billet)}×100.  (4)

The steel bar or wire rod after finish rolling needs not to be cooled to the room temperature at a cooling rate not more than that of spontaneous cooling. After the surface temperature of the steel bar or wire rod reaches not more than 600° C., the steel bar or wire rod may be cooled at a cooling rate more than that of spontaneous cooling, such as by air cooling, mist cooling, water cooling, and the like.

The above described heating temperature represents an average value of in-furnace temperature of a heating furnace. The above described heating time represents the time period in a furnace at the above described heating temperature. The finishing temperature represents the surface temperature of the steel bar or wire rod just after finish rolling. The finish rolling represents, for example, rolling in the final stand among a plurality of stands used for rolling in a continuous mill. The cooling rate after finish processing represents the surface cooling rate of the steel bar or wire rod.

An example of the method for producing a machine part by using a rolled steel bar or wire rod for hot forging is as follows.

A rolled steel bar or wire rod for hot forging is subjected to hot forging to produce an intermediate product having a rough shape. The intermediate product may be subjected to thermal refining treatment. The thermal refining treatment is for example normalizing. The intermediate product is machined into a predetermined shape. The machining is for example cutting or piercing.

The intermediate product after machining may be subjected to a casehardening treatment. The casehardening treatment is, for example, carburizing treatment, nitriding treatment, or induction quenching treatment, etc. The casehardened intermediate product is subjected to finish processing to produce a machine part.

The steel bar or wire rod which has been produced by the above described processes has excellent bending fatigue strength, surface fatigue strength, and wear resistance as well as excellent machinability even after hot forging.

Example 1

Steels A to C having chemical compositions shown in Table 1 were melted by a 70-ton converter.

TABLE 1 Chemical composition (by mass %, balance being Fe and impurities) Steel C Si Mn P S Cr Mo Al Ti N O fn1 A 0.21 *0.21 0.86 0.012 0.013 *1.08 — 0.029 0.001 0.0157 0.0012 *1.08 B 0.21 *0.19 0.78 0.013 0.014 *1.02 *0.18 0.031 0.002 0.0161 0.0011 *1.38 C 0.15 0.46 0.63 0.012 0.014 1.81 — 0.028 0.001 0.0163 0.0010 1.81 *indicates being out of the range of the present invention

The molten steels of Steels A to C were used to produce cast pieces (blooms) of 400 mm×300 mm by a continuous casting process. The produced blooms were spontaneously cooled to 600° C. in the atmosphere. Note that the cast piece in the course of solidification was subjected to rolling reduction in a continuous casting step.

Next, production conditions shown in Table 2 were set.

TABLE 2 Cast piece Billet Rolling condition Heating Heating Water-cooling Finishing Condition temperature Heating time temperature Heating time before temperature number (° C.) (min) (° C.) (min) finish rolling (° C.) Cooling condition 1 1280 120 1200 90 Without 970 Spontaneous cooling 2 1280 720 1200 90 Without 970 Spontaneous cooling 3 1280 720 1200 90 With 900 Spontaneous cooling 4 1280 720 1200 120 Without 970 Water cooling to 800° C., thereafter spontaneous cooling 5 1280 240 1200 40 Without 930 Spontaneous cooling 6 1280 720 1270 120 Without 1050 Spontaneous cooling 7 1280 720 1150 90 Without 900 Spontaneous cooling 8 1200 720 1200 90 Without 970 Spontaneous cooling

Specifically, a “heating temperature” column in a “cast piece” column of Table 2 shows heating temperatures (° C.) of the cast piece at each condition. A “heating time” column in the “cast piece” column of Table 2 shows heating times (minutes) of the cast piece at each condition. Similarly, a “heating temperature” column in a “billet” column of Table 2 shows heating temperatures (° C.) of the billet at each condition. A “heating time” column in the “billet” column shows heating times (minutes) of the billets at each condition. A “water cooling before finish rolling” column in a “rolling condition” column shows whether or not water cooling of the billet was performed before finish rolling at each condition. “With” in the column indicates that water cooling was performed. And “without” indicates that water cooling was not performed. A “finishing temperature” column in the “rolling condition” column shows finishing temperatures (° C.) at each condition. “cooling condition” column in the “rolling condition” column shows cooling conditions after finish rolling at each condition.

Steel bars of Test numbers 1 to 10 shown in Table 3 were produced based on the steels shown in Table 1 and the production conditions shown in Table 2.

TABLE 3 Ratio of Cutting test/ average Medium High cycles/ Surface Amount of Table 2/ ferrite cycles/Bending Bending fatigue Amount of major cutting Test Production grain fatigue strength fatigue strength strength wear edge wear number Classification Steel condition Microstructure size (Normalized) (Normalized) (Normalized) (Normalized) (Normalized) 1 Comparative *A 2 F + P 1.5 Reference (100) Reference Reference Reference 70 (100) (100) (100) 2 Comparative *B 2 F + P + B 1.7 #105 #105 120 80 Reference (100) 3 Comparative C 1 F + P + B #2.3 #112 #108 120 80 75 4 Inventive C 2 F + P + B 1.7 120 120 130 70 70 5 Comparative C 3 F + P + B #2.2 #114 #108 120 80 75 6 Comparative C 4 F + P + B #2.8 #102 #105 #115 #85 75 7 Comparative C 5 F + P + B #2.5 #106 #110 120 80 75 8 Comparative C 6 F + B #3.3 #101 #108 #105 #95 75 9 Inventive C 7 F + P + B 1.3 125 115 130 70 70 10 Comparative C 8 F + P + B #2.1 #114 115 125 75 70 *indicates being out of the range of the present invention #indicates that the target of the present invention has not been reached

Specifically, in each Test number, the cast pieces of the steels shown in Table 3 were heated at the production conditions (heating temperature and heating time of cast piece) shown in Table 3. The heated cast piece was billeted to produce a billet of 180 mm×180 mm. The produced billet was cooled to the room temperature (25° C.).

Next, the billet was heated at the production conditions (heating temperatures and hating times of the billet) shown in Table 3. The heated billet was hot rolled at the production conditions (water cooling before finish rolling, finishing temperature, cooling condition) shown in Table 3 to produce steel bars having a diameter of 50 mm and a diameter of 70 mm. The steel bars after rolling were spontaneously cooled as-is to the room temperature in the atmosphere. That is, the steel bars were as-hot rolled members.

[Microstructure Observation Test]

A steel bar of 50 mm diameter was cut perpendicular to its longitudinal direction. A sample including a central portion of the cut section was cut out. In the surfaces of the sample, the surface corresponding to the above described central portion was polished into a mirror surface. And the polished surface was etched with Nital. The etched surface was observed in 15 visual fields with an optical microscope of 400-times magnification. The 15 visual fields were arbitrarily selected from a region excluding any decarburized layer in the outer layer. The size of each visual field was 250 μm×250 μm. Microstructure was observed in each visual field.

As a result of the microstructure observation test, the microstructure of any Test number did not include martensite. The microstructure of each Test number was any one of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, and a ferrite-bainite structure. The results of the microstructure observation are shown in a “microstructure” column in Table 3. “F+P” in the table indicates that the microstructure of corresponding Test number was a ferrite-pearlite structure. And “F+P+B” indicates a ferrite-pearlite-bainite structure, and “F+B” indicates a ferrite-bainite structure.

[Average Ferrite Grain Size Measurement]

Average ferrite grain sizes of the above described 15 visual fields were measured according to the intercept method specified in JIS G0551 (2005).

Among the average ferrite grain sizes (15 in total) of each visual fields, a maximum value and a minimum values were determined. Then, a ratio of average ferrite grain size (=maximum value/minimum value) was determined based on Formula (3). The ratios of average ferrite grain size are shown in Table 3.

[Preparation of Surface Fatigue Strength Specimen and Bending Fatigue Strength Specimen]

The steel bar of each Test number was heated at 1200° C. for 30 minutes. Next, the steel bar was subjected to hot forging with the finishing temperature being not less than 950° C. to produce a round bar having a diameter of 35 mm. The round bar of 35 mm diameter was machined to prepare a small roller specimen for roller pitting (hereafter, simply referred to as a small roller specimen) shown in FIG. 1, and a notched Ono-type rotating bending fatigue specimen shown in FIG. 2 (the dimensional unit in the drawing is mm for both FIGS. 1 and 2). The small roller specimen shown in FIG. 1 included a test portion (a columnar portion having a diameter of 26 mm and a width of 28 mm) in its center.

Each prepared specimen was subjected to carburizing and quenching at the conditions shown in FIG. 3 by using a gas carburizing furnace. After quenching, the specimen was subjected to tempering at 150° C. for 1.5 hours. The small roller specimen and the Ono-type rotating bending fatigue specimen were subjected to finish processing of the gripping portion for the purpose of removing heat treatment strain.

[Surface Fatigue Strength Test]

In the roller pitting test, the above described small roller specimen and a large roller having a shape shown in FIG. 4 (the dimensional unit in the drawing is mm) were combined. The large roller shown in FIG. 4 was made of steel which satisfied the specification of SCM420H in JIS, and was fabricated through general production processes, that is, processes of normalizing, specimen machining, eutectic carburizing with a gas carburizing furnace, low temperature tempering and polishing.

A roller pitting test by use of the small roller specimen and the large roller was conducted at conditions shown in Table 4.

TABLE 4 Test machine Roller pitting test machine Specimen Small roller of 26 mm dia. Large roller of 130 mm dia. (contact portion 700 mmR) Maximum interfacial pressure 4000 MPa Number of tests Six Slip factor −40% Number of revolutions of 1000 rpm small roller Circumferential speed Small roller: 1.36 m/s, Large roller: 1.90 m/s Lubricant oil temperature 90° C. Oil used Oil for automatic transmission

As shown in Table 4, the number of revolutions of the small roller specimen was 1000 rpm, a slip factor was −40%, a contact interfacial pressure between the large roller and the small roller specimen during test was 4000 MPa, and the number of repetition was 2.0×10⁷ cycles. When the rotational speed of the large roller was V1 m/sec and the rotational speed of the small roller specimen was V2 m/sec, the slip factor (%) was determined by the following formula:

Slip factor=(V2−V1)/V2×100.

During testing, a lubricant (commercially available oil for automatic transmission) was sprayed at an oil temperature of 90° C. to the contact portion (surface of the test portion) between the large roller and the small roller specimen in the counter direction to the rotational direction. The roller pitting test was conducted at the conditions described above to evaluate the surface fatigue strength.

For each Test number, six tests were conducted in the roller pitting test. After testing, an S-N curve was created in which contact stress was plotted for the ordinate and the number of repetition until the occurrence of pitting was plotted for the abscissa. Among the contact stress of the specimens in which pitting did not occur until the number of repetition of 2.0×10⁷, the highest contact stress was defined as the surface fatigue strength of that Test number. It was defined that pitting occurred when among portions of the small roller specimen where the surface was damaged, the area of the largest portion reached not less than 1 mm².

Table 3 shows surface fatigue strengths obtained by the test. In the surface fatigue strength of Table 3, the surface fatigue strength of Test number 1 was set as a reference value (100%). Then, the surface fatigue strength of each Test number was represented by a ratio (%) with respect to the reference value. If the surface fatigue strength was not less than 120%, it was judged that excellent surface fatigue strength was obtained.

[Wear Resistance Evaluation]

The amount of wear of the test portion of the small roller specimen was measured for the specimens in which the number of repetition in the roller pitting test reached 1.0×10⁶. Specifically, a maximum height roughness (Rz) was determined according to JIS B0601 (2001). Here, a smaller Rz value indicates a higher wear resistance. A surface roughness tester was used to measure the amount of wear. Table 3 shows the amounts of wear. In the amount of wear in Table 3, the amount of wear of Test number 1 was set as a reference value (100%). Then, the amount of wear of each Test number was represented by a ratio (%) with respect to the reference value. When the amount of wear was not more than 80%, it was judged that excellent wear resistance was obtained.

[Bending Fatigue Strength Test]

The bending fatigue strength was determined by the Ono-type rotating bending fatigue test. The number of tests in the Ono-type rotating bending fatigue test was decided to be eight for each Test number. The test was conducted with the number of revolutions during testing being 3000 rpm, and otherwise in an ordinary manner. Among the stresses of the specimens which have not broken off until the numbers of repetition of 1.0×10⁴ cycles and 1.0×10⁷ cycles, the highest stresses were defined as a medium-cycle and high-cycle rotating bending fatigue strengths, respectively.

Medium-cycle and high-cycle bending fatigue strengths are shown in Table 3. In the medium-cycle and high-cycle bending fatigue strengths, the medium-cycle and high-cycle bending fatigue strengths of Test number 1 were set as reference values (100%). Then, the medium-cycle and high-cycle bending fatigue strengths of each Test number were represented by ratios (%) with respect to the reference values. It was judged that excellent bending fatigue strength was obtained if the bending fatigue strength was not less than 115% for both the medium cycle and high cycle.

[Cutting Test]

A cutting test was conducted to evaluate machinability. A cutting specimen was obtained by the following method. A steel bar of 70 mm diameter of each Test number was heated at a heating temperature of 1250° C. for 30 minutes. The heated steel bar was hot forged at a finishing temperature of not less than 950° C. to obtain a round bar of 60 mm diameter. From this round bar, a cutting specimen having a diameter of 55 mm and a length of 450 mm was obtained by machining. By using this cutting specimen, a cutting test was conducted at the following conditions.

Cutting Test (Lathe Turning)

Tip: Base metal material is P20 grade cemented carbide without coating

Test conditions: Circumferential speed is 200 m/min, feed is 0.30 mm/rev, a depth of cut is 1.5 mm, with the use of water-soluble cutting oil

Measured item: Amount of major cutting edge wear of flank face after 10 minutes of cutting time

Table 3 shows obtained amounts of major cutting edge wear. In Table 3, the amount of major cutting edge wear of Test number 2 (Steel, B was used) was set as a reference value (100%). Then, the amount of major cutting edge wear of each Test number was represented by a ratio (%) with respect to the reference value. It was judged that excellent machinability was obtained when the amount of major cutting edge wear was not more than 80%.

[Evaluation Results]

With reference to Table 3, the chemical compositions of the steel bars of Test numbers 4 and 9 (Steel C) were within the range of the present invention, and fn1 satisfied Formula (2). Further, the ratios of average ferrite grain size of Test numbers 4 and 9 were both not more than 2.0. As a result, for Test number 4 and 9, the medium-cycle and high-cycle bending fatigue strengths were not less than 115%, and the surface fatigue strength was not less than 120%. Further, the amount of wear was not more than 80%. Furthermore, the amount of major cutting edge wear was not more than 80%. Therefore, steel bars of Test numbers 4 and 9 exhibited excellent bending fatigue strength, surface fatigue strength, wear resistance and machinability.

On the other hand, the chemical composition of the steel bar of Test number 1 (Steel A) corresponded to SCr420H in JIS. For that reason, the Si content and the Cr content of Test number 1 were less than the lower limits of the Si content and the Cr content of the present invention. Further, fn1 of Test number 1 was less than the lower limit of Formula (2). As a result, Test number 1 exhibited low bending fatigue strength, surface fatigue strength, and wear resistance.

The chemical composition of the steel bar of Test number 2 (Steel B) corresponded to SCM420H in JIS. Therefore, the Si content and the Cr content of Test number 2 were less than the lower limits of the Si content and the Cr content of the present invention. Further, the Mo content of Test number 2 exceeded the upper limit of the Mo content of the present invention. Further, fn1 of Test number 2 was less than the lower limit of Formula (2). As a result, the bending fatigue strength of Test number 2 was as low as less than 115%, and the machinability thereof was also low.

The chemical composition of Test number 3 (Steel C) was within the range of the chemical composition of the present invention. Further, fn1 satisfied Formula (2) as well. However, since the heating time of the cast piece was too short (see Production condition 1 in Table 2), the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths of Test number 3 were as low as less than 115%.

The chemical composition of Test number 5 was within the range of the present invention, and also fn1 satisfied Formula (2). However, in Test number 5, water cooling was performed before finish rolling (see Production condition 3 in Table 2). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths of Test number 5 were as low as less than 115%.

The chemical composition of Test number 6 was within the range of the chemical composition of the present invention, and also fn1 satisfied Formula (2). However, in Test number 6, the steel bar after finish rolling was subjected to water cooling to 800° C. (see Production condition 4 in Table 2). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, both the medium-cycle and high-cycle bending fatigue strengths of Test number 6 were as low as less than 115%. Further, the surface fatigue strength was as low as less than 120%. Furthermore, the amount of wear exceeded 80%, indicating low wear resistance.

The chemical composition of Test number 7 was within the range of the chemical composition of the present invention, and also fn1 satisfied Formula (2). However, in Test number 7, the heating time of the cast piece was too short, and also the heating time of the billet was too short (see Production condition 5). As a result, the ratio of average ferrite grain size exceeded 2.0. For that reason, both the medium-cycle and high-cycle bending fatigue strengths of Test number 7 were as low as, less than 115%.

The chemical composition of Test number 8 was within the range of the chemical composition of the present invention, and also fn1 satisfied Formula (2). However, in Test number 8, the heating temperature of the billet was too high, and also the finishing temperature thereof was too high (see Production condition 6). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, both the medium-cycle and high-cycle bending fatigue strengths of Test number 8 were as low as less than 115%. Further, the surface fatigue strength was as low as less than 120%. Furthermore, the amount of wear exceeded 80%, indicating low wear resistance.

The chemical composition of Test number 10 was within the range of the chemical composition of the present invention, and also fn1 satisfied Formula (2). However, in Test number 10, the heating temperature of the cast piece was too low (see Production condition 8). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle bending fatigue strength was as low as less than 115%.

Example 2

Molten steels having the chemical compositions of D to S shown in Table 5 were produced in the same manner as in Example 1.

TABLE 5 Chemical composition (by mass %, balance being Fe and impurities Steel C Si Mn P S Cr Mo Al Nb Ti N O fn1 D 0.22 *0.21 0.81 0.014 0.013 *1.35 0.03 0.033 — 0.002 0.0148 0.0009 *1.41 E 0.18 0.33 0.75 0.007 0.011 1.53 — 0.031 — 0.001 0.0177 0.0011 *1.53 F 0.21 *0.63 0.82 0.013 0.013 1.68 0.07 0.035 — 0.001 0.0165 0.0012 1.82 G 0.20 0.33 0.82 0.014 0.015 1.90 0.05 0.030 — 0.002 0.0151 0.0010 2.00 H 0.21 0.36 0.79 0.015 0.013 1.82 — 0.030 — 0.001 0.0139 0.0011 1.82 I 0.18 0.49 0.63 0.013 0.015 1.92 0.09 0.031 — 0.001 0.0182 0.0011 2.10 J 0.22 0.51 0.75 0.011 0.014 2.00 — 0.034 — 0.001 0.0175 0.0009 2.00 K 0.20 0.48 0.71 0.013 0.015 *2.13 — 0.035 — 0.001 0.0155 0.0013 *2.13 L 0.21 0.36 0.69 0.012 0.013 *1.40 0.08 0.037 — 0.002 0.0152 0.0011 *1.58 M 0.19 0.52 0.75 0.015 0.013 1.82 *0.18  0.034 — 0.001 0.0162 0.0011 *2.18 N 0.21 0.35 0.80 0.012 0.013 1.85 0.06 0.037 0.031 0.002 0.0182 0.0009 1.97 O 0.23 0.42 0.72 0.011 0.012 1.98 — 0.038 0.034 0.001 0.0176 0.0012 1.98 P 0.19 0.37 *0.45 0.013 0.013 1.81 — *0.021 — 0.002 0.0155 0.0010 1.81 Q 0.22 0.41 *1.10 0.012 0.013 1.87 — *0.056 — 0.001 0.0149 0.0009 1.87 R 0.21 0.41 0.81 0.012 0.014 1.94 0.09 *0.056 — 0.001 0.0149 0.0009 *2.12 S 0.17 0.34 0.76 0.008 0.011 1.53 0.05 0.028 — 0.001 0.0181 0.0011 1.63 *indicates deviation from the range of the present invention

Then, the steel bars of Test numbers 11 to 42 shown in Table 6 were produced in the same production conditions as in Example 1. The diameters of the steel bars were 50 mm and 70 mm. By using the produced steel bars, the same tests as in Example 1 were conducted. Then, the medium-cycle and high-cycle bending fatigue strengths, surface fatigue strength, wear resistance, and the amount of major cutting edge wear were determined, respectively.

TABLE 6 Cutting test/ Ratio of Medium High Surface Amount of Table 2/ average cycles/Bending cycles/Bending fatigue Amount of major cutting Test Production ferrite fatigue strength fatigue strength strength wear edge wear number Classification Steel condition Microstructure grain size (Normalized) (Normalized) (Normalized) (Normalized) (Normalized) 11 Comparative *D 2 F + P + B 1.6 118 120 #115 #95 75 12 Comparative *D 1 F + P + B *2.2 #104 #106 #115 #85 75 13 Comparative *E 2 F + P + B 1.8 116 #114 120 80 80 14 Comparative *E 3 F + P *2.2 #107 #110 120 80 80 15 Comparative *F 2 F + P + B 1.6 #114 #105 120 80 #105 16 Comparative *F 4 F + P + B *2.3 #108 #100 #115 #85 #105 17 Inventive G 2 F + P + B 1.6 118 120 120 75 70 18 Comparative G 1 F + P + B *2.2 #112 116 #115 80 70 19 Inventive H 7 F + P 1.3 120 120 130 70 70 20 Comparative H 3 F + P + B *2.3 #112 #114 120 75 70 21 Inventive I 2 F + B 1.7 126 123 140 60 80 22 Comparative I 4 F + P + B *2.6 #114 #113 125 75 80 23 Inventive J 7 F + P + B 1.6 126 122 140 60 80 24 Comparative J 5 F + P + B *2.4 #114 115 135 65 80 25 Comparative *K 7 F + P + B 1.7 126 124 135 65 #105 26 Comparative *K 5 F + P + B *2.5 #114 #113 130 70 #105 27 Comparative *L 2 F + B 1.7 #112 #114 #115 70 80 28 Comparative *L 6 F + B *2.7 #110 #113 #110 75 80 29 Comparative *M 2 F + B 1.7 122 120 125 75 #110 30 Comparative *M 8 F + B *2.5 #112 #104 120 80 #110 31 Inventive N 7 F + P + B 1.4 128 124 130 70 75 32 Comparative N 6 F + B *2.4 #114 118 125 75 80 33 Inventive O 2 F + P + B 1.4 128 122 135 65 75 34 Comparative O 8 F + P + B *2.2 #114 116 125 75 75 35 Comparative *P 2 F + P + B 1.7 #112 115 #115 70 70 36 Comparative *P 4 F + P + B *2.6 #110 #108 #115 80 70 37 Comparative *Q 2 F + P + B 1.7 118 #107 130 70 #110 38 Comparative *Q 6 F + P + B *2.3 #108 #105 125 75 #110 39 Comparative *R 2 F + P + B 1.8 126 123 140 60 #110 40 Comparative *R 3 F + P + B *2.6 #112 #114 120 75 #110 41 Inventive S 2 F + P + B 1.8 116 117 120 80 80 42 Inventive S 7 F + P 1.7 116 116 120 80 80 *indicates being out of the range of the present invention #indicates that the target of the present invention has not been reached

Obtained results are shown in Table 6. Referring to Table 6, the chemical compositions of Test numbers 17, 19, 21, 23, 31, 33, 41 and 42 were within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). Further, all of the ratios of average ferrite grain size of these Test numbers were not more than 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths of these Test numbers were not less than 115%, and the surface fatigue strengths thereof were not less than 120%. Furthermore, the amounts of wear were not more than 80%. Furthermore, the amounts of major cutting edge wear were not more than 80%.

On the other hand, the Si content and the Or content of the chemical composition of Test number 11 (Steel D) were less than the lower limits of the Si content and the Or content of the present invention. As a result, the surface fatigue strength of Test number 11 was less than 120%, and the amount of wear was more than 80%. Test number 12 used the same Steel D as with Test number 11. As a result, the surface fatigue strength and the wear resistance were low. Further, in Test number 12, the heating time of the cast piece was too short (Production condition 1). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%.

Although the chemical composition of Test number 13 (Steel E) was within the range of the chemical composition of the present invention, fn1 was less than the lower limit of Formula (2). As a result, the high-cycle bending fatigue strength was as low as less than 115%. Test number 14 used the same Steel E as with Test number 13. For that reason, the high-cycle bending fatigue strength was low. Further, in Test number 14, water cooling was performed before finish rolling (Production condition 3). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were lower than those of Test number 13.

The Si content of the chemical composition of Test number 15 (Steel F) exceeded the upper limit of the Si content of the present invention. As a result, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%. Further, the amount of major cutting edge wear was more than 80%, indicating low machinability.

Test number 16 used the same Steel F as with Test number 15. As a result, the bending fatigue strength and the machinability were low. Further, in Test number 16, the steel bar after finish rolling was water-cooled to 800° C. (Production condition 4). As a result, the ratio of average ferrite grain size exceeded 2.0. For that reason, Test number 16 showed a lower bending fatigue strength than that of Test number 15. Moreover, the surface fatigue strength thereof was less than 120%, and the amount of wear thereof was more than 80%.

The chemical composition of Test number 18 (Steel G) was within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). However, the heating time of the cast piece was too short (Production condition 1). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle bending fatigue strength was as low as less than 115%. Further, the surface fatigue strength was as low as less than 120%.

The chemical composition of Test number 20 (Steel H) was within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). However, water cooling was performed before finish rolling (Production condition 3). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%.

The chemical composition of Test number 22 (Steel I) was within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). However, the steel bar after finish rolling was water cooled to 800° C. (Production condition 4). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%.

The chemical composition of Test number 24 (Steel J) was within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). However, the heating time of the cast piece and the heating time of the billet were too short (Production condition 5). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle bending fatigue strength was as low as less than 115%.

The Cr content of the chemical composition of Test number 25 (Steel K) exceeded the upper limit of the Cr content of the present invention. For that reason, the amount of major cutting edge wear was more than 80%, indicating low machinability. It was inferred that an excessive Cr content caused excessive production of bainite in steel.

Test number 26 used the same Steel K as with Test number 25. As a result, the machinability thereof was low. Further, in Test number 26, the heating time of the cast piece and the heating time of the billet were too short (Production condition 5). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%.

The Cr content of the chemical composition of Test number 27 (Steel L) was less than the lower limit of the Cr content of the present invention. For that reason, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%. Further, the surface fatigue strength was as low as less than 120%.

Test number 28 used the same Steel L as with Test number 27. For that reason, the bending fatigue strength was low. Further, in Test number 28, the heating temperature of the billet was too high, and the finishing temperature was also too high (Production condition 6). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%. Further, the surface fatigue strength was as low as less than 120%.

The Mo content of the chemical composition of Test number 29 (Steel M) exceeded the upper limit of the Mo content of the present invention. As a result, the amount of major cutting edge wear of Test number 29 exceeded 80%, indicating low machinability. It was inferred that an excessive Mo content caused excessive production of bainite in steel.

Test number 30 used the same Steel M as with Test number 29. As a result, the machinability was low. Further, in Test number 30, the heating temperature of the cast piece was too low (Production condition 8). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were as low as less than 115%.

The chemical composition of Test number 32 (Steel N) was within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). However, the heating temperature and finishing temperature of the billet were too high (Production condition 6). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle bending fatigue strength was as low as less than 115%.

The chemical composition of Test number 34 (Steel O) was within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). However, the heating temperature of the cast piece was too low (Production condition 8). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle bending fatigue strength was as low as less than 115%.

The Mn content and the Al content of the chemical composition of Test number 35 (Steel P) were less than the lower limits of the Mn content and the Al content of the present invention. For that reason, the medium-cycle bending fatigue strength was as low as less than 115%. Further, the surface fatigue strength was as low as less than 1200.

Test number 36 used the same Steel P as with Test number 35. As a result, the medium-cycle bending fatigue strength and the surface fatigue strength were low. Further, in Test number 35, the steel bar after finish rolling was water-cooled to 800° C. (Production condition 4). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the high-cycle bending fatigue strength was as low as less than 115%. Further, the medium-cycle bending fatigue strength was lower than that of Test number 35.

The Mn content and the Al content of the chemical composition of Test number 37 (Steel Q) exceeded the upper limits of the Mn content and the Al content of the present invention. For that reason, the high-cycle bending fatigue strength was as low as less than 115%. Further, the amount of major cutting edge wear exceeded 80%, indicating low machinability.

Test number 38 used the same Steel Q as with Test number 37. As a result, the high-cycle bending fatigue strength was low and also the machinability was low. Further, in Test number 38, the heating temperature and the finishing temperature of the billet were too high. For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle bending fatigue strength was as low as less than 115%. Further, the high-cycle bending fatigue strength was lower than that of Test number 37.

Although the chemical composition of Test number 39 (Steel R) was within the range of the chemical composition of the present invention, fn1 exceeded the upper limit of Formula (2). As a result, the machinability of the steel of Test number 39 was low. Test number 40 used the same Steel R as with Test number 39. For that reason, the machinability of the steel of Test number 40 was low. Further, in Test number 40, water cooling was performed before rolling (Production condition 3). For that reason, the ratio of average ferrite grain size exceeded 2.0. As a result, the medium-cycle and high-cycle bending fatigue strengths were lower than those of Test number 39.

Although the embodiments of the present invention have been described so far, the above described embodiments are merely examples to carry out the present invention. Therefore, the present invention is not limited to the above described embodiments, and can be carried out by appropriately modifying the above described embodiments within a range not departing from the spirit of the present invention. 

1. A rolled steel bar or wire rod for hot forging, comprising: a chemical composition comprising, by mass %, C: 0.1 to 025%, Si: 0.30 to 0.60%, Mn: 0.50 to 1.0%, S: 0.003 to 0.05%, Cr: 1.50 to 2.00%, Mo: not more than 0.10% (including 0%), Al: 0.025 to 0.05%, N: 0.010 to 0.025%, and the balance being Fe and impurities, wherein the impurities contain P: not more than 0.025%, Ti: not more than 0.003%, and O (oxygen): not more than 0.002%, respectively, and wherein fn1 defined by Formula (1) is 1.60 to 2.10; and a structure consisting of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure, wherein a maximum value/a minimum value of average ferrite grain size, which is obtained by making a measurement in 15 visual fields each having an area of 62500 μm² in a cross section, is not more than 2.0: fn1=Cr+2×Mo  (1) where each symbol of elements in Formula (1) is substituted by a content (mass %) of a corresponding element.
 2. The rolled steel bar or wire rod for hot forging according to claim 1, wherein fn1 is not less than 1.80.
 3. The rolled steel bar or wire rod for hot forging according to claim 1, further comprising, by mass %, Nb: not more than 0.08% in place of a part of Fe.
 4. The rolled steel bar or wire rod for hot forging according to claim 2, further comprising, by mass %, Nb: not more than 0.08% in place of a part of Fe. 